The invention relates to a device for the exact positioning of a printing head in relation to a recording substrate in a printer.
In a printer, recording substrates in the form of single sheets which consist of paper or plastic are transported in a recording substrate feed shaft. From the recording substrate feed shaft, the recording substrates are fed to a recording substrate holder, on which they are printed.
The recording substrate holder contains means which bring the recording substrate into a desired position on the recording substrate holder. The desired position can, for example, be achieved when one edge of the recording substrate bears on a recording substrate stop which is provided at the edge of the recording substrate holder. If the desired position is fixed in two dimensions, a second recording substrate stop, which is provided at right angles to the first recording substrate stop on a further side boundary of the recording substrate holder, can fix the desired position of the recording substrate in this second position.
Now, if the recording substrate lies with its edges on the recording substrate stops, the printing process can be carried out under the precondition that the position of the recording substrate holder in relation to the printing head and the format of the recording substrate are known.
Normally, printers process recording substrates in a standard format or the format respectively being used must be preselected by am operator. A further possibility of determining the format of the recording substrate consists in guiding a sensor, operating according to the light reflection principle, over the recording substrate holder. In this arrangement, use is made of the different reflection properties of the recording substrate and of the recording substrate holder. Unprinted recording substrates reflect the light much more strongly than the surface of the recording substrate holder. However, problems always arise with this reflection light barrier if a recording substrate has already been printed in part regions before the printing process, that is to say, for example, contains a form which is intended to be filled out by means of the current printing process, or if the reflective capability of the recording substrate differs from common recording substrates, for example because of a particular coloration of its entire surface or of its edges.
The assignment of the recording substrate holder to the printing head is effected by means of a reduction in the geometric tolerances between the printing head mechanism and the recording substrate holder. This tolerance Japanese reference JP 3-197161 discloses a device for positioning a printing head using different reflection properties of a paper, a reflection rail and an end marker. A photosensor and a printing head are arranged together in a moving carrier.
Japanese reference JP 3-142270 discloses to deposit a recording substrate on a recording substrate holder and to determine the position of the recording substrate by means of sensors operating according to the reflection principle. The sensors are arranged together with a printing head in a carrier. PA1 European reference EP 0 406 236 B1 discloses an optical measuring device by means of which the lateral deposition of a recording substrate can be determined. On a displaceable carrier, a transmitting and receiving element are arranged opposite each other in such a manner that a recording substrate which is brought into their beam path is detected. PA1 IBM Technical Disclosure Bulletin, Vol. 28, No. 4, September 1985, pages 1681-1683, discloses an arrangement for determining the position of a printed sheet. The arrangement operates according to the transmission principle, an LED being provided as transmitting means and a row of photo-transistors as receiving means. The printed sheet is guided between transmitting and receiving means and the position is determined by scanning the phototransistors. The distance between the individual phototransistors is limited to a minimum distance by the size of the components and, for this reason, the position of the printed sheet can be determined only with a relatively coarse resolution, reduction is complicated and expensive.
The present invention is then based on the object of providing a device for the exact positioning of a printing head in relation to a recording substrate in a printer, which makes it possible to assign the position of the printing head and of the printable region of the recording substrate exactly to each other, in order that the printed characters can be imaged with high reliability at the desired positions.
In general terms the present invention is a device for the exact positioning of a printing head in relation to a recording substrate in a printer.
The recording substrate is deposited on a recording substrate holder and is aligned on at least one recording substrate stop bounding the recording substrate holder on one side. The printing head is moveable in at least one direction transverse to the recording substrate stop over the recording substrate holder. At least one receiving or transmitting means which can be moved together with the printing head is coupled in a defined position with the printing head. At least one transmitting or receiving means, which extends in the movement direction of the printing head, is arranged in or on the recording substrate holder in such a way that the transmitting or receiving means can be covered over at least partially by the recording substrate. As a result, a communication between the receiving or transmitting means and the transmitting or receiving means may be interrupted in the deposit region of the recording substrate.
Advantageous developments of the present invention are as follows. The recording substrate holder has a shielding surface which is arranged at a defined distance from the recording substrate stop and in the region of the extent of the transmitting or recording means. The shielding surface is suitable for interrupting communication between the receiving or transmitting means and the transmitting or receiving means.
The transmitting or receiving means assigned to the recording substrate holder is arranged underneath the recording substrate holder. The recording substrate holder is formed by a flat plate which is transparent, at least in the region of the extent of the transmitting or receiving means.
The transmitting means assigned to the recording substrate holder has a plurality of light-emitting diodes which are lined up as an LED row and arranged on a flat subassembly. Between the recording substrate holder and the LED row, a light conducting means is provided. The light conducting means is formed by a shaft which is connected in one piece to the recording substrate holder. The light-emitting diodes are arranged at such a distance from one another that the radiation cones of adjacent light-emitting diodes partially overlap at the surface of the recording substrate holder.
The receiving or transmitting means which can be moved together with the printing head is assigned a slit diaphragm.
In the region of the surface of the recording substrate holder, a refraction means is provided that deflects the beam path by an angle between receiving and transmitting means.
To align the recording substrate, at least one further transmitting means and one further receiving means are provided. The further transmitting means is arranged on one side of a recording substrate holder and the further receiving means is arranged on the other side of the recording substrate holder such that the main reception direction of the receiving means is located in the region of influence of a main emission direction of the transmitting means. The recording substrate can be brought between the transmitting means and the receiving means. The axis of the main emission direction of the transmitting means is at an oblique angle to the recording substrate holder. The transmitting means is fastened below the recording substrate holder to the flat plate. The receiving means is fastened in a cover over the recording substrate holder. The cover can be pivoted about a pivot into a locking position in which the axis of the main emission direction of the transmitting means and of the main reception direction of the receiving means align.
The distance of an edge of the recording substrate, lying opposite the recording substrate stop, from the recording substrate stop and hence the format of the recording substrate is not determined according to the reflection principle but according to the transmission principle. This improves the reliability of the format detection. The direct assignment of the receiving or transmitting means to the printing head makes possible a positioning of the printing head within close tolerance limits.
According to a further development and refinement of the invention, the recording substrate holder has a shielding surface which is arranged at a defined distance from the recording substrate stop and in the region of the extent of the transmitting or receiving means, and which is suitable for interrupting a communication between the receiving means or transmitting means and the transmitting or receiving means. The printing head is guided transversely to the recording substrate stop over the recording substrate holder, proceeding from a rest position. In the course of this movement, the transmitting and receiving means which are assigned to the printing head and to the recording substrate holder communicate with each other. For example, one side emits light, while the other side receives this light. A change in the light intensity is detected by the receiving means and communicated to a control unit.
Proceeding from the rest position, the printing head, and hence also its transmitting or receiving means, passes into the region of influence of the shielding surface. During the transition from the shielding surface to the non-shielded part of the recording substrate holder, a message is accordingly given to the control unit. Since the distance between the recording substrate stop and the shielding surface is now known, conclusions can now be drawn about the mutual position of the printing head and the recording substrate holder. In the course of the movement of the printing head, both the exact position of the edge of the recording substrate resting on the recording substrate stop and the edge, lying opposite the recording substrate stop, of the recording substrate are accordingly known in terms of their exact position. The control unit is therefore capable of guiding the printing head with high reliability to exactly those positions at which characters or the like are intended to be printed.
According to a further development and refinement of the invention, the transmitting or receiving means assigned to the recording substrate holder is arranged underneath the recording substrate holder. The recording substrate holder is in this case formed by a flat plate which is transparent to the radiation proceeding from the transmitting means, at least in the region of the extent of the transmitting or receiving means. As a result, no apertures or depressions, which can accommodate the transmitting or receiving means, need to be introduced into the recording substrate holder. In the case of a flat plate which has no apertures or depressions, a deposit of recording substrate residues in the apertures or depressions cannot occur. Rather, a self-cleaning effect occurs as a result of the recording substrate being moved over the flat plate. The functional reliability of the device is thereby improved.
According to a further development and refinement of the invention, a guiding means is provided between the recording substrate holder and the transmitting or receiving means assigned to the recording substrate holder. This guiding means enables the arrangement of the transmitting or receiving means at a distance from the recording substrate holder and additionally effects a restriction of the cross section of the beam path between the transmitting and receiving means. The beams from the transmitting means can consequently be directed exactly onto the receiving means. Particularly good guiding properties are achieved by means of a single-piece connection of the light guiding means to the recording substrate holder. The exact alignment of the radiation is also optimized.
According to a further development and configuration, the recording substrate holder has assigned to it as transmitting means a plurality of light-emitting diodes which are lined up as an LED row and are arranged on a flat subassembly. By means of the arrangement on the flat subassembly, cost-effective production by means of automatic population is made possible. An alignment of the individual light-emitting diodes in the direction of the light guiding means needs to be carried out only within wide tolerance limits. These tolerance limits can be maintained in the case of automatic population without subsequent machining. A light-emitting diode has an emission angle of .+-.20.degree., while the light guiding means detects an emission region of only .+-.10.degree. from the light-emitting diodes and forwards it to the surface of the recording substrate holder. The light guiding shaft accordingly ensures that, in the case of an oblique position of the LEDs of .+-.10.degree. no impairment of the functioning occurs.
By means of an arrangement of the light-emitting diodes at such a distance from one another that the radiation cones of adjacent light-emitting diodes partially overlap at the surface of the recording substrate holder, on the one hand a uniform light intensity at the surface of the recording substrate holder is achieved and, on the other hand, the failure of a single light-emitting diode can be compensated by the directly adjacent serviceable light-emitting diodes.